The present invention relates to color image processing apparatus, and more particularly relates to a color image processing apparatus for performing detection and correction of pixel defects suitably for a single-sensor color sensor image or a like color image which is composed of a pixel array of different colors.
The rate of occurrence of defect pixels, i.e., fault pixels is generally higher and technology for detecting and correcting these is indispensable in such image input apparatus as a high-definition camera where a solid-state image pickup device having a large number of pixels is used. By correcting pixels at singularities represented by these fault pixels, a higher yield can be achieved of the solid-state image pickup device to greatly reduce price of the apparatus.
The known technologies for electrically correcting such fault pixels include the following techniques. Specifically, in a technique disclosed for example in Japanese patent laid-open applications Hei-5-236358, Hei-7-162757, Hei-10-322603, a memory is provided for previously storing and retaining locations of fault pixels which occur uniquely to each device at the time of fabrication of the solid-state image pickup devices. The memory is mounted on an image input device such as camera so as to extrapolate the fault pixels at predetermined locations by means of average values, etc., obtained from adjoining pixels while continuously surveying output signals from the memory.
Further, Japanese patent laid-open applications Hei-6-205302, Hei-6-245148, disclose a technique in which defects are detected and corrected in real time during readout of signals from the pixels without providing a memory. In particular, a pixel to be observed and nearby four consecutive pixels are used so that, when the observed pixel stands out in the surrounding pixel signals by more than a certain value while at the same time the preceding and succeeding adjoining pixel signals are at certain level or above, the observed pixel is determined as defective and is corrected.
Furthermore, Japanese patent laid-open application Hei-6-284346 discloses a solid-state image pickup apparatus using a color filter of mosaic array of complementary colors in which stored to a memory for a plurality of fields are the results of comparison between detection output signal of level difference of pixel signals of identical color and a threshold value. Fault pixels are determined based on such information retained in the memory, thereby making it possible to accurately detect the fault pixels even in the presence of edge components and to perform correction among pixels of identical color by using surrounding pixels of different colors.
Moreover, in a defect correcting apparatus for color image pickup device as disclosed in Japanese patent laid-open application Hei-10-126795, signal of fault pixel regarding its luminance signal is corrected by adjoining pixels irrespective of the color of the pixels, while it is corrected by signals of nearby pixels of identical color regarding its color signals. The defect due to difference in hue is thereby made less conspicuous at the same time of the defect in luminance.
The one disclosed in the above mentioned Japanese patent laid-open applications Hei-5-236358, Hei-7-162757, Hei-10-322603 requires an exclusive memory for every individual sensor, i.e., each camera. It cannot be diverted to another camera. It is also incapable of dealing with those defects which occur due to a change with the passage of time after the shipping of camera from factory. In addition, there is a problem that capacity of the memory for storing the locations of defect is increased in proportion to the increase in number of pixels resulting from higher definition of image, increasing both price and power consumption.
On the other hand, the one disclosed in Japanese patent laid-open applications Hei-6-205302, Hei-6-245148, while not requiring a memory for storing the locations of defect, is capable of dealing with a black-and-white sensor, since detection of fault pixels using consecutive pixels is premised. If, however, the technique disclosed in these publications is applied to a color sensor or to an image without a consecutive occurrence of identical color, there is a problem that detection errors are caused due to the difference in sensitivity to specific color among the adjoining pixels of different colors. When, in order to avoid this, an attempt is made to detect fault pixels by using pixels of identical color, space of unit block for detection becomes very large in a color sensor without successions of identical color and a weaker correlation results among the pixels which are not spatially consecutive. For this reason, there is also a problem that detection errors tend to occur.
Further, while capable of accurately detecting fault pixels even in the presence of edge components, the one disclosed in Japanese patent laid-open application Hei-6-284346 requires for such purpose a memory for storing the results of comparison of level difference and threshold value for a plurality of fields, making it impossible to detect defects in real time. Also, while it is designed to correct defects of an identical color by operation of surrounding pixels of different colors, it is limited to those sensors having thereon a mosaic filter of complementary colors. Furthermore, the one disclosed in Japanese patent laid-open application Hei-10-126795 is with a problem that correlation among adjoining pixels cannot be obtained and erroneous detection of defects tends to occur, since defects regarding color signals are detected after all by using correlation among unconsecutive pixels of identical color which are separated from each other by more than one pixel.